Ultraviolet-curing composition, optical disk, and method of producing optical disk

ABSTRACT

To obtain a digital video disc having excellent durability in a high temperature and high humidity environment, an ultraviolet curable composition prepared such that water in which the cured coating film has been immersed has a specific conductivity of 100 μS/cm or less is provided as an adhesive for use in bonding two discs with information recording layers, respectively, having thin films of different kinds of material.

TECHNICAL FIELD

The present invention relates to an adhesive used when bonding twodiscs, at least one of which has an information recording layer, to eachother, and more particularly relates to an ultraviolet curablecomposition.

The present application is based on Japanese Patent Application No. Hei10-147000 and Japanese Patent Application No. Hei 10-147001, thecontents of which are incorporated herein by reference.

BACKGROUND ART

Optical discs having a bonded structure which have hitherto been wellknown include, for example, magneto optical discs capable of recordingand reproducing information. A magneto optical disc has a transparentsubstrate of polycarbonate, etc., on which an information recordingmaterial composed of, for example, a TbFeCo alloy, etc., and provided onboth sides thereof with a protective layer composed of silicon nitride,etc., is laminated. An information recording layer is formed by furtherforming on the outermost layer a thin film mainly composed of aluminumas a reflecting layer for a laser beam for reading information. Ifneeded, a protective layer composed of an ultraviolet curable resin isfurther formed on the thin film composed mainly of aluminum. In onecase, this is used as it is as a single plate, and in another case, thesingle plates are bonded via an adhesive such that the informationrecording layers oppose each other, before they are used.

As the adhesive, a hot-melt type adhesive and an adhesive composed of anultraviolet curable resin are used.

Thus, an optical disc having a conventional bonded structure in whichboth of the two discs have an information recording layer is of such astructure that “thin films of the same kinds of metal” composed mainlyof aluminum oppose each other via an adhesive. Japanese PatentPublications Nos. 2553171 and 2555178 disclose technologies relative toultraviolet curable adhesives suitable for the optical discs.

A digital video disc or digital versatile disc (DVD), a new type ofoptical disc having a bonded structure, is produced by a method ofbonding two discs of which at least one disc has an informationrecording layer, and use of an ultraviolet curable composition as anadhesive is being studied.

In the case of read-only DVDs, the structure of the bonded twosubstrates may be one of the following types or the like, which can beselected properly depending on the application: a type in whichpolycarbonate substrates are used which are each provided on one surfacethereof with irregularities called pits which correspond to informationto be recorded and further, as a reflecting film for a laser beam forreading information there, an aluminum layer, for example, is formed, toprovide an information recording layer (DVD-10); a type in which one ofsuch substrates is formed with a semitransparent film of gold, siliconnitride or the like to provide an information recording layer (DVD-9); atype in which as one of the substrates a transparent polycarbonatesubstrate having no information recording layer is used (DVD-5); and atype in which each substrate has two information recording layerscomposed of a layer of aluminum, etc., and a semitransparent film ofgold, silicon nitride or the like, and two such substrates are bonded(DVD-17).

Of these, a DVD-9 disc is of a structure in which “thin films ofdifferent kinds of material” such as aluminum and gold or siliconnitride are opposed through an adhesive. When the ultraviolet curablecompositions as described in Japanese Patent Publications Nos. 2553171and 1555178 are used as an adhesive, the durability of the discs isinsufficient, and they are very weak in adhesive force to the gold orsilicon nitride in the semitransparent film, so that the compositionsdescribed in the above patents are poor in serviceability as an adhesivefor DVDs.

When gold is used as a semitransparent film for DVD-9, the structurebecomes such that “different types of metal” are opposed to each otherthrough an adhesive.

Different metals, when immersed in aqueous solutions, have intrinsicpotentials, and the contact of or the electrical connection of metalshaving different potentials results in formation of a battery, and themetal having a lower potential is corroded. This is well known and thedetails thereof are described in, for example, “Denki Kagaku Hou(Electrochemical Method) (published by Kodansha).”

The standard electrode potentials of various compounds in aqueoussolutions are described in detail in, for example, Denki Kagaku Binran(Manual of Electrochemistry) (ed. Denki Kagaku Kyokai (ElectrochemicalAssociation), published by Maruzen Co., Ltd.); for the Al³⁺/Al reactionon an aluminum electrode, the potential is −1.68 V, and for the Au³⁺/Aureaction on an aluminum electrode, the potential is +1.50V.

Therefore, for example, when a bonded disc of the DVD-9 type having thinfilms of aluminum and of gold is placed in a high temperature and highhumidity environment, under the condition in which these metal thinfilms are electrically connected to each other, the corrosion of thealuminum, which has a lower potential, is promoted. This is specific toDVD-9 discs, which have metal thin films of different types.Accordingly, the prior art adhesive for optical discs directed to discshaving thin films of the same kinds of metal composed mainly of aluminumhas not been designed taking such electrochemical corrosion mechanismsinto consideration, so that there has been the problem of insufficientdurability when the DVD-9 discs are used without modification.

That is, a problem to be solved by the present invention is to providean ultraviolet curable composition having excellent durability in DVD-9discs by making a design capable of inhibiting the phenomenon ofcorrosion specific to not only the case where a semitransparent filmcomposed of an inorganic compound such as silicon nitride is used butalso to the case where a semitransparent film of a metal such as gold isused and to provide an ultraviolet curable composition used as anadhesive for optical discs which can be used advantageously for all theformats including DVD-5, DVD-10, and DVD-17.

DISCLOSURE OF THE INVENTION

The present inventors have carried out intensive study to solve theabove problem, and, as a result, have found that when the specificconductivity of pure water in which a cured coating film of anultraviolet curable composition has been immersed is equivalent to orbelow a specified value, the durability of the bonded disc increases,and that this effect is particularly remarkable in DVD-9 discs havingthin films of different kinds of metal, and thus the present inventorsachieved the present invention.

That is, the present invention provides an ultraviolet curablecomposition for optical discs, the composition containing an ultravioletcurable compound and a photopolymerization initiator and being used whenbonding two substrates which have information recording layers,respectively, on each outermost layer of which a thin film of differenttype of material is formed, wherein the ultraviolet curable compositionis characterized in that water in which a cured coating film has beenimmersed has a specific conductivity of 100 μS/cm or less.

BEST MODE FOR CARRYING OUT THE INVENTION

In optical discs of the present invention, the specific conductivity ofwater in which a cured coating film, which corresponds to a curedadhesive, has been immersed is defined as follows.

First, about 1 g of a cured coating film is peeled and is weighedexactly to four decimal places.

Then, 100 g of pure water is put in a 250 ml brown polymer bottle, andfurther the cured coating film weighed as above is added. As the purewater, pure water which is initially adjusted to 18 MΩ•cm is used.

Further, the polymer bottle containing this sample is placed in an ovenat 80° C. and taken out after 96 hours, and the specific conductivity(A) of water in which the cured coating film has been immersed ismeasured at 25±1° C.

The calculations are carried out according to the following:${{Specific}\quad {conductivity}} = \frac{{specific}\quad {conductivity}\quad (A)}{{weight}\quad {of}\quad {cured}\quad {coating}\quad {film}}$

Specific conductivity was measured using a CM-50AT conductivity metermanufactured by TOA Electronics, Ltd.

In the case of commercially available optical discs, their specificconductivity can be measured according to the above, and in the casewhere an ultraviolet curable composition (adhesive) before it is usedfor bonding is available, the specific conductivity of water in whichthe cured coating film has been immersed according to the presentinvention can be measured also by the following procedure.

(1) A glass plate is coated with an ultraviolet curable composition to afilm thickness of about 100 μm. As the glass plate, one having no effecton the measurement of specific conductivity is used.

(2) In the case where the ultraviolet irradiation method is bycontinuous light irradiation, an MO3-L31 (120 W/cm metal halide lampwith a cold mirror) manufactured by EYE GRAPHICS CO., LTD. is used as aUV lamp under a nitrogen atmosphere and irradiation of 500 mJ/cm²(ultraviolet light actinometer: UVPF-36 manufactured by EYE GRAPHICSCO., LTD.) is carried out at a lamp height of 10 cm to prepare a curedcoating film.

(3) In the case where the ultraviolet irradiation method is by flashirradiation, a flash irradiation apparatus equipped with 4 lamps is usedunder a nitrogen atmosphere, which is adjusted such that input energyper shot per lamp is 200 J, and ultraviolet light is irradiated at afrequency of 2 Hz by 10 shots to prepare a cured coating film.

The lamp used here is as follows, and the actual output energy from thelamp for the above input energy is as follows. The emission spectrum offlash irradiation light from the lamp is measured using aspectroradiometer (USR-20A manufactured by Ushio Inc.) and integratedfrom 300 nm to 390 nm to obtain the ultraviolet light strength andirradiation amount, which are adjusted such that the ultraviolet lightstrength is 19 W/cm² and the ultraviolet irradiation amount is 12mJ/cm².

(4) The cured coating film peeled off from the glass plate is reduced insize to about 1 to 2 cm square and about 1 g is weighed exactly to fourdecimal places.

(5) In a 250 ml brown polymer bottle, 100 g of pure water is added, andfurther the cured coating film weighed in (4) is added. As the purewater, pure water which is initially adjusted to 18 MΩ•cm is used.

(6) The polymer bottle containing this sample is placed in an oven at80° C. and taken out after 96 hours and the specific conductivity (A) ofwater in which the cured coating film has been immersed measured at25±1° C.

(7) The calculations are carried out according to the following:$\text{Specific~~~conductivity of~~~the present invention} = \frac{{specific}\quad {conductivity}\quad (A)}{{weight}\quad {of}\quad {cured}\quad {coating}\quad {film}}$

(8) The specific conductivity is measured using a CM-50A conductivitymeter manufactured by TOA Electronics, Ltd.

The specific conductivity of water in which the cured coating film hasbeen immersed depends on the amount of water-soluble electrolytecomponents eluted from the cured coating film. Therefore, uponpreparation of the composition, it is preferred to select those whichcontain no (absolutely none) or minimal amounts of components which canbe water-soluble electrolyte components, such as reaction catalysts andcarboxylic acids represented by (meth)acrylic acid, which remain inpolymerizable monomers such as monofunctional (meth)acrylate andpolyfunctional (meth)acrylate, or in polymerizable oligomers such aspolyester acrylate, polyether acrylate, epoxy acrylate, and urethaneacrylate, and various additives added as thermal polymerizationinhibitors, antioxidants, etc.

Further, since the photopolymerization initiator decomposates which aregenerated upon irradiation with ultraviolet light or side reactionproducts derived from the decomposates can also be electrolytecomponents, it is preferred that the photopolymerization initiators beselected so that these components occur in small amounts.

The raw materials for preparing the composition of the present inventionare purified before use, as needed.

As for the acrylates as described below, it is often the case that upontheir synthesis the following catalysts or additives are used forpreventing polymerization, and since they act as water-solubleelectrolytes which increase the specific conductivity in the presentinvention, those which have as a small water-soluble electrolyte contentas possible are used.

For example, in the case where acrylic acid and an alcohol or alkyleneglycol are directly reacted to synthesize polyalkylene glycolpolyacrylate, there are used, for example, sulfuric acid, concentratedsulfuric acid, phosphoric acid, etc., so that commercially availableproducts may contain them in relatively large amounts.

It is sometimes the case that commercially available polymerizablemonomers and polymerizable oligomers contain hydroxylamine,phenylenediamine used as a polymerization inhibitor in relatively largeamounts.

For example, in the case where acrylic acid is reacted withalkylene(ethylene oxide or propylene oxide) to synthesize β-hydroxyester, there are used, for example, iron compounds such as ferricchloride, chromium compounds such as chromic anhydride, alkali metalcompounds such as sodium carbonate, tertiary amines, quaternary ammoniumsalts, quaternary pyridinium salts, etc., so that commercially availableproducts may contain these in relatively large amounts.

In the case of synthesizing alkyl aminoacrylate by transesterificationbetween methyl acrylate and alkylamino alcohol, there are used, forexample, sodium methylate, di-n-butyltin oxide, etc., orparahydroxydiphenylamine, phenothiazine, diethylhydroxyamine, etc., forinhibiting anionic polymerization, so that commercially availableproducts may contain these in relatively large amounts.

In the case where an adduct of an alcohol and ε-caprolactone is used asa raw material upon synthesis of acrylates, the catalyst used upon theaddition reaction of ε-caprolactone in many cases is contained as it isas an impurity. In this case, the catalyst contained in the above adductas a commercially available product includes, for example,organotitanium compounds such as tetraisopropyl titanate and tetrabutyltitanate, tin compounds such as tetraphenyltin, tetraoctyltin,dilauryltin oxide, and di-n-butyltin dichloride.

Further, it is often the case that among the monomers, there areimpurities derived from reaction by-products or unreacted components.

Further, it is often the case that the polymerizable monomers andpolymerizable oligomers when acrylic acid is used contain unused acrylicacid or polymerisate of acrylic acid. It is often the case that, forbenzyl acrylate, an example of acrylates having a benzene ring, itscommercially available product produced from sodium acrylate and benzylchloride contains sodium chloride. It is often the case that whenacrylic acid and ethylene oxide are reacted, the resulting ethyleneglycol is included. It is often the case that when desalting is carriedout in order to separate this, ammonium chloride or the like isincluded.

Upon preparing the composition of the present invention, each of thepolymerizable oligomer and polymerizable monomer constituting thecomposition used are those which contain as little water-solubleelectrolyte components as possible.

The main feature of the present invention is that the composition isadjusted so that the specific conductivity of water in which the curedcoating film has been immersed is finally 100 μS/cm or less. By doingso, even in high temperature and high humidity environments, theabove-described contact corrosion of different types of metal can beinhibited so that the durability of DVD-9 discs increases markedly.

The specific conductivity of water in which the cured coating film hasbeen immersed may be 100 μS/cm or less but is preferably 50 μS/cm orless since contact corrosion can be more effectively inhibited. That is,the specific conductivity of water in which the cured coating film hasbeen immersed may be 0 to 100 μS/cm but is preferably 0 to 50 μS/cmsince the contact corrosion can be more sufficiently inhibited.

Upon preparing the ultraviolet curable composition of the presentinvention, an ultraviolet curable compound and a photopolymerizationinitiator are used as essential components. As the ultraviolet curablecompound, monofunctional (meth)acrylate and polyfunctional(meth)acrylate can be used as a polymerizable monomer component. Thesemay be used singly or two or more of them may be used in combination. Inthe present invention, acrylate and methacrylate together are called(meth)acrylate and similarly, acrylic acid and methacrylic acid togetherare called (meth)acrylic acid.

In the present invention, taking into consideration the physicalproperties of the cured coating film, the necessary polymerizablemonomers and polymerizable oligomers are selected from those below andcombined to prepare the composition of the present invention. It issatisfactory if each of the polymerizable monomers and polymerizableoligomers selected for the preparation of the composition is in theabove specific conductivity range but the composition is adjusted suchthat the composition as a whole is in the above specific conductivityrange.

The polymerizable monomer which can be used in the present inventionincludes, for example, the following.

The monofunctional (meth)acrylate includes, for example, (meth)acrylateand the like, having a group such as methyl, ethyl, propyl, butyl, amyl,2-ethylhexyl, octyl, nonyl, dodecyl, hexadecyl, octadecyl, cyclohexyl,benzyl, methoxyethyl, butoxyethyl, phenoxyethyl, nonylphenoxyethyl,tetrahydrofurfuryl, glycidyl, 2-hydroxyethyl, 2-hydroxypropyl,3-chloro-2-hydroxypropyl, dimethylaminoethyl, diethylaminoethyl,nonylphenoxyethyltetrahydrofurfuryl, caprolactone-modifiedtetrahydrofurfuryl, isobornyl, dicyclopentanyl, dicyclopentenyl,dicyclopentenyloxyethyl or a similar group as a substituent group.

Also, the polyfunctional (meth)acrylate includes, for example,diacrylates of 1,3-butylene glycol, 1,4-butanediol, 1,5-pentanediol,3-methyl-1,5-pentanediol, 1,6-hexanediol, neopentyl glycol,1,8-octanediol, 1,9-nonanediol, tricyclodecanedimethanol, ethyleneglycol, polyethylene glycol, propylene glycol, tripropylene glycol,polypropylene glycol, etc., di(meth)acrylate oftris(2-hydroxyethyl)isocyanurate, di(meth)acrylate of a diol obtained byadding 4 moles or more of ethylene oxide or propylene oxide to 1 mole ofneopentyl glycol, di(meth)acrylate of a diol obtained by adding 2 molesof ethylene oxide or propylene oxide to 1 mole of bisphenol A, di- ortri(meth)acrylate of a triol obtained by adding 3 moles or more ofethylene oxide or propylene oxide to 1 more of trimethylolpropane,di(meth)acrylate of a diol obtained by adding 4 moles or more ofethylene oxide or propylene oxide to 1 more of bisphenol A,tris(2-hydroxyethyl) isocyanurate tri(meth)acrylate, trimethylolpropanetri(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritolpoly(meth)acrylate, caprolactone-modified tris[(meth)acryloxyethyl]isocyanurate, alkyl-modified dipentaerythritol poly(meth)acrylate,caprolactone-modified dipentaerythritol poly(meth)acrylate,hydroxypivaric acid neopentyl glycol diacrylate, caprolactone-modifiedhydroxypivalic acid neopentyl glycol diacrylate, ethylene oxide-modifiedphosphoric acid (meth)acrylate, ethylene oxide-modified alkylatedphosphoric acid (meth)acrylate, etc.

Also, N-vinyl-2-pyrrolidone, acryloylmorpholine, vinylimidazole,N-vinylcaprolactam, N-vinylformamide, vinyl acetate, (meth)acrylic acid,(meth)acrylamide, N-hydroxymethylacrylamide, orN-hydroxyethylacrylamide, and alkyl ether compounds thereof, etc., canalso be used.

Further, those which can be used in combination similarly to thepolymerizable monomer include as polymerizable oligomers, polyester(meth)acrylate, polyether (meth)acrylate, epoxy (meth)acrylate, urethane(meth)acrylate, etc.

In the present invention, the photopolymerization initiator may be anypublicly known, commonly used one with which ultraviolet curablecompounds represented by the polymerizable monomers and/or polymerizableoligomers used can be cured. As the photopolymerization initiator, thosewhich have a molecular extinction coefficient of 50 (1/mol•cm) or moreat 370 to 450 nm, for example, and molecule cleaving type or hydrogeneliminating type ones that do not belong thereto are suitable for thepresent invention. Use of these in combination is more suitable.

Of the photopolymerization initiators used in the present invention,those having a molecular extinction coefficient of 50 (l/mol•cm) or moreat 370 to 450 nm, for example, include benzoin isobutyl ether,2,4-diethylthioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone,benzil, 2,4,6-trimethylbenzoyldiphenylphosphine oxide,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone-1,bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, etc.,and further, as molecule cleaving type ones other than these,1-hydroxycyclohexyl phenyl ketone, benzoyl ethyl ether, benzyl dimethylketal, 2-hydroxy-2-methyl-1-phenyl-propan-1-one,1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one and2-methyl-1-(4-methylthiophenyl)-2-morpholino-propan-1-one, etc., may beused in combination. Further, benzophenone, 4-phenylbenzophenone,isophthalophenone, 4-benzoyl-4′-methyl-diphenyl sulfide, etc., which arehydrogen eliminating type photopolymerization initiators, may be used incombination. As the molecule cleaving type photopolymerization initiatorused in combination with one having a molecular extinction coefficientof 50 (l/mol•cm) or more at 370 to 450 nm is preferred1-hydroxycyclohexyl phenyl ketone with respect to curability and thedurability of bonded discs.

In combination with the above photopolymerization initiator can be used,as a sensitizer, amines that do not undergo addition polymerizationreaction with the above-described polymerizable components, such as, forexample, triethylamine, methyldiethanolamine, triethanolamine,p-diethylaminoacetophenone, p-dimethylaminoacetophenone, ethylp-dimethylaminobenzoate, isoamyl p-dimethylaminobenzoate,N,N-dimethylbenzylamine, and 4,4′-bis(diethylamino)benzophenone. Ofcourse, it is preferred to select for use the above-describedphotopolymerization initiators and sensitizers from those which areexcellent in solubility in the curable components and which do notinterfere with ultraviolet light transmission.

It is preferred to use ultraviolet curable compositions which are liquidat room temperature to 40° C. It is preferred not to use solvents. Ifused, it is preferred to limit the amount to a level as small aspossible. In the case where coating of the above-described compositionis done using a spin coater, it is preferred to adjust the viscosity to20 to 1,000 mPa•s and in the case where a relatively thick film is to beobtained, it may be adjusted to 100 to 1,000 mPa•s.

To the composition of the present invention may if necessary be blendedas still other additives thermal polymerization inhibitors, antioxidantsrepresented by hindered phenols, hindered amines, phosphites, etc.,plasticizers, silane coupling agents represented by epoxysilane,mercaptosilane, (meth)acrylsilane, etc., and the like in order toimprove various properties. These are selected for use from those whichare excellent in solubility in curable components, which do notinterfere with ultraviolet transmission, and which contain no orminimized amounts, if any, of water-soluble electrolyte components.

Upon preparing the composition of the present invention, the chemicalcomposition is appropriately selected such that the physical propertiesof not only the above-described photopolymerization initiator but alsoof the composition itself and the physical properties of curable bondinglayer are suitable for a DVD.

Since two substrates are bonded in the case of a DVD, if it is droppedby mistake, the substrate may be cracked or peeled off from the adhesionsurface. To prevent this, it is preferred that the composition of thepresent invention be prepared such that the modulus of elasticity of thecured coating film is 1 to 1,000 MPa, preferably 1 to 500 MPa at 30° C.

As for the ultraviolet curable composition of the present invention, itis preferred that not only the specific conductivity of water in whichthe cured coating film has been immersed be set in a specified range butalso that the water absorption of cured coating film be set to 1 to 7 wt%. If the water absorption is below 1 wt %, then after removing the discfrom a high temperature and high humidity tester, bubbles readily occurbetween the adhesive cured coating film and the disc surface contactingthe cured coating film so that deformation occurs in the signal layer oradhesive cured film, tending to lead to an increase in error in readingrecorded information. Water absorption exceeding 7 wt % is notpreferable since pin-holes tend to be formed in the thin film of ametal, such as, for example, aluminum.

In this case, the water absorption of the cured coating film can bemeasured as follows.

First, according to (1) to (3) of the measurement of specificconductivity of water in which the cured coating film has been immersed,a cured coating film is prepared similarly and the water absorption ofthe cured coating film is measured according to the method B of JISK7209-1.

The ultraviolet curable composition thus adjusted is used, in thepresent invention, when bonding two substrates each having aninformation recording layer thereon, on which thin films of differentkinds of material are formed. In the present invention, two substrates,on each of which an information recording layer is laminated whichcontains a thin film of a metal or inorganic compound, are opposed toeach other and integrated to achieve adhesion. The excellent technicaleffect of the present invention is exhibited particularly remarkablywhen the respective metals or inorganic compounds constituting the twothin films to be bonded to each other are a combination of differentkinds of material which generate a difference in potential in a standardelectrode potential.

In the present invention, a substrate to be bonded to another has aninformation recording layer including a thin film layer provided byforming a thin film, and the entirety of a portion obtained therefrom byexcluding the substrate itself is called an information recording layer.The outermost layer of the information recording layer means thefollowing light reflecting layer and semitransparent film. As thesubstrate, a heat resistant thermoplastic resin transparent substrate istypical.

In these two heat resistant thermoplastic resin transparent substrateswhich are to be bonded, on which information recording layers arelaminated, respectively, including thin films as the outermost layersincluded in the respective information recording layers, that is, thelight reflecting layer and the semitransparent film, the metalsconstituting such layers include, for example, aluminum and metalscomposed mainly of aluminum, and gold and the inorganic compoundsconstituting such layers include, for example, silicon based inorganiccompounds such as silicon nitride, silicon carbide or silicon. Thecombination of different kinds of material that will cause a differencein potential in the standard electrode potential includes, for example,a combination of a metal composed mainly of aluminum-gold (lightreflecting film-semitransparent film). The light reflecting film is notparticularly limited in its material if it has a reflectivity of 45% ormore for a laser beam for reading information while the semitransparentfilm is not particularly limited in its material if it has areflectivity of 18% or more for a laser beam for reading information. Atany rate, in the combination of different kinds of material that willcause a difference in potential in the standard electrode potential, theeffect of the present invention is exhibited remarkably.

When each piece to be bonded is viewed from the transparent substrateside (that is, when the transparent substrate is the innermost layer),the light reflecting film and semitransparent film in the informationrecording layers are the outermost layers in the entire informationrecording layers. The outermost layers are bonded together by curingwith the ultraviolet curable composition so that they contact each otherdirectly, with the interlayer space between the light reflecting filmand the semitransparent film being directly joined by the cured film ofthe composition.

In short, using the two substrates comprising information recordinglayers, on each outermost layer of which the thin film of different typeof material is formed, the ultraviolet curable composition according tothe present invention is provided between the thin films; they arearranged to oppose each other; they are bonded together; and thereafter,ultraviolet light is irradiated through either one thin film or both thethin films to convert the composition to a cured coating film tointegrate both substrates with each other.

Thus obtained is an optical disc having at least a bonded structurecomprising two substrates having information recording layers,respectively, on each outermost layer of which a thin film of differenttype of material is formed, the thin films in the respective substratesbeing bonded to each other through a cured coating film of anultraviolet curable composition containing an ultraviolet curablecompound and a photopolymerization initiator, wherein water in which thecured coating film has been immersed has a specific conductivity of 100μS/cm or less.

In the present invention, as described above, the thin films asoutermost layers of the exposed information recording layers may bebonded directly or as the case may be, protective coat layers forprotecting the thin films may be provided and the protective coat layersmay be bonded to each other. In this case, the protective coat layersare formed usually from a cured product of an ultraviolet curablecomposition containing an ultraviolet curable compound and aphotopolymerization initiator. In this case, the protective coat layersmust be transparent to ultraviolet light. The ultraviolet curablecomposition for the protective coat layers is preferably prepared sothat it is excellent in adhesion to both the outermost exposed layer ofthe information recording layer and the cured product of the ultravioletcurable composition. Even in the case where the substrates provided witha protective coat layer on the outermost layer of the informationrecording layer are bonded, the protective coat layer is in awater-absorbed state under high temperature and high humidityenvironment and hence the water-soluble electrolyte components in theadhesive cured coating film can transmit through the protective coatlayer. Therefore, even when the composition of the present invention isnot in contact with the outermost layer of information recording layerdirectly, the effect of using the composition of the present inventionis exhibited so that a DVD-9 disc which is excellent in durability canbe obtained.

The optical disc provided with the above-described protective coat layercan be obtained by providing the ultraviolet curable composition of thepresent invention on at least one or both of thin films each of which isformed on one of the two substrates comprising information recordinglayers, on each outermost layer of which the thin film of different typeof material is formed; irradiating ultraviolet light onto thecomposition to provide a protective coat layer composed of a curedcoating film of the ultraviolet curable composition; then providing anultraviolet curable composition, which may or may not be of the typeaccording to the present invention, containing an ultraviolet curablecompound and a photopolymerization initiator between the protective coatlayer of one substrate and the thin film of the other substrate opposingeach other, or between the protective coat layers of both substratesopposing each other; bonding them together; and irradiating ultravioletlight through either one of or both of the thin films to convert thecomposition into a cured coating film to integrate both substrates witheach other.

The optical disc provided with the above-described protective coat layermay be of two types in layer construction.

(1) An optical disc having at least a bonded structure comprising twosubstrates having information recording layers, respectively, on eachoutermost layer of which a thin film of different type of material isformed, on one of the thin films a protective coat layer composed of acured coating film which transmits to ultraviolet light being provided,the coating film being obtained from an ultraviolet curable compositioncontaining an ultraviolet curable compound and a photopolymerizationinitiator, the protective coat layer of one of the substrates and thethin film of the other of the substrates being bonded together by thecured coating film of the ultraviolet curable composition containing anultraviolet curable compound and a photopolymerization initiator,wherein the optical disc is characterized in that a water in which thecured coating film of the protective coat layer has been immersed has aspecific conductivity of 100 μS/cm or less.

(2) An optical disc having at least a bonded structure comprising twosubstrates having information recording layers, respectively, on eachoutermost layer of which a thin film of different type of material isformed, on each of the thin films a protective coat layer composed of acured coating film which transmits to ultraviolet light being provided,the coating film being obtained from an ultraviolet curable compositioncontaining an ultraviolet curable compound and a photopolymerizationinitiator, the protective coat layers of the both substrates beingbonded together by the cured coating film of the ultraviolet curablecomposition containing an ultraviolet curable compound and aphotopolymerization initiator, wherein the optical disc is characterizedin that water in which the cured coating film of the protective coatlayer has been immersed has a specific conductivity of 100 μS/cm orless.

In this case, it is preferred to use the above-described composition ofthe present invention in bonding a protective coat layer and a thin filmor in bonding a protective coat layer and a protective coat layer.

The technical effect of the present invention is exhibited not only in aDVD-9 disc having the above-described layer structure but also in aDVD-17 disc corresponding to a layer structure in which two of the DVD-9discs are bonded to each other.

As the optical discs having at least a structure comprising twosubstrates having information recording layers, respectively, on eachoutermost layer of which a thin film of different type of material isformed, the thin films on the two substrates being bonded together bythe cured coating film of the ultraviolet curable composition containingan ultraviolet curable compound and a photopolymerization initiator,there are optical discs having a set of thin films of different types ofmaterial (two sheets of thin film) and optical discs having two sets ofthin films of different types of material (four sheets of thin film),regardless of whether or not the protective coat layer is present. Theoptical disc having a set of the thin films of different kinds ofmaterial is a DVD-9 disc.

The optical disc in the case where there are two sets of thin films ofdifferent kinds of material corresponds to DVD-17. The optical disc inthe case where there are two sets of thin films of different kinds ofmaterial can be obtained by bonding two optical discs each having oneset of the thin films of different kinds of material such that for bothdiscs, the thin film having a higher ultraviolet transmittance of thetwo thin films in each disc is arranged on the outer side.

The composition of the present invention can be also used for DVD-RAMsof the rewritable type or DVD-R of the writable type.

As the heat resistant thermoplastic transparent resin substrate, therecan be used, for example, polycarbonate, polymethyl methacrylate,amorphous polyolefins, etc.

In the present invention, two heat resistant thermoplastic transparentresin substrates, on each of which an information recording layer islaminated, are bonded to integrate them such that their respective thinfilm sides are opposed to each other, by irradiating ultraviolet lightthrough either one thin film or both the thin films toward thecomposition of the present invention to cure it. The irradiation ofultraviolet light is preferably performed from the side of the thin filmhaving a higher ultraviolet transmittance if the irradiation withultraviolet light is performed only to either one of the thin films.

The optical discs having the protective coat layer as in (1) to (2)described above can be obtained by providing an ultraviolet curablecomposition containing an ultraviolet curable compound and aphotopolymerization initiator, such that water in which an ultraviolettransmitting cured coating film has been immersed has a specificconductivity of 100 μS/cm or less, between thin films on two substratescomprising information recording layers, on each outermost layer ofwhich the thin film of different type of material is formed; irradiatingultraviolet light onto the composition to provide a protective coatlayer composed of a cured coating film of the ultraviolet curablecomposition; then providing an ultraviolet curable compositioncontaining an ultraviolet curable compound and a photopolymerizationinitiator between the protective coat layer of one substrate and thethin film of the other substrate opposing each other, or between theprotective coat layers of both substrates opposing each other; bondingthem together; and irradiating ultraviolet light through either one ofor both of the thin films to convert the composition into a curedcoating film to integrate both substrates with each other.

Upon curing the composition of present invention with ultraviolet light,as the continuous light irradiation system which is publicly known, acommonly used system can be used, for example, metal halide lamp, highpressure mercury lamp, etc.

However, a flash irradiation system is more preferred in that theultraviolet curable composition can be cured sufficiently with a smalleramount of energy than the conventional continuous light irradiationsystem and that by using the composition having the same chemicalcomposition, the specific conductivity of water in which the curedcoating film has been immersed can be made lower (In the presentinvention, this irradiatoin system is referred to as flash ultravioletirradiation system).

In the present invention, the flash irradiation of ultraviolet light isperformed so that one flash is completed in a very short time on theorder of from microseconds to milliseconds. To fabricate one bondeddisc, the flash occurs once, or is repeated, and preferably flashirradiation is carried out 3 to 15 times.

In flash ultraviolet irradiation, there can be used publicly known,commonly used ultraviolet light sources, for example, metal halidelamps, high pressure mercury lamps, xenon lamps, xenon-mercury lamsp,etc. Examples of lamps having sufficient durability to endure repeatedemission of flashes carried out in the present invention include a xenonlamp.

Since use of the above-described ultraviolet light source as it isresults in the emitted ultraviolet light being a continuous light (i.e.,the system is a continuous light irradiation system), a flashing lightemission apparatus is formed by combining it with a dischargingmechanism for flash irradiation to enable flash ultraviolet irradiation.As the flash discharging mechanism, there can be used, for example, acircuit having serially connected a capacitor for condensing the charge,a coil for controlling the current waveform upon discharging, theabove-described light source and an electrode.

By conducting flash ultraviolet irradiation on the composition of thepresent invention by using a combination of the ultraviolet light sourceand flash discharging mechanism, bonding by curing can be performed witha reduced power consumption and in a shorter irradiation time ascompared with those required for bonding by curing using theabove-described continuous light irradiation system, while suppressingcurling to a very low level without damaging the information recordinglayer.

Upon conducting flash ultraviolet irradiation, it is preferred that aninfrared light blocking filter be provided between the above-describedsubstrate and the ultraviolet light source so that the composition in anuncured state between the thin films can be bonded by curing whileblocking the infrared light in order to obtain a final product DVD discwith less curl.

Besides DVD, there have been conventionally, for example, a magnetooptical disc (MO) having one recording film of a laminate structure forone-sided recording, such as polycarbonate/silicon nitride/TbFeCo(recording layer) /silicon nitride/aluminum reflecting layer/protectivelayer for an ultraviolet curable resin film and a magneto optical dischaving two recording films for double-sided recording of a laminatestructure such as polycarbonate/silicon nitride/TbFeCo (recordingfilm)/silicon nitride/aluminum reflecting layer/ultraviolet curableadhesive/aluminum reflecting layer/silicon nitride/TbFeCo (recordingfilm) /silicon nitride/polycarbonate.

However, in the former, in the interlayer space between the layers ofdifferent kinds of metal or inorganic compounds there is no ultravioletcurable resin, and in the latter the ultraviolet curable adhesive incontact with the same metal (aluminum) and sandwiched thereby.Therefore, in either case, there is no possibility that the standardelectrode potentials are differentiated

Therefore, the effect of the present invention is particularlyremarkable, and cannot be expected by merely applying to DVD-9 anultraviolet curable protective coating agent for a light reflecting filmfor a conventional one sided recording MO or an ultraviolet curableadhesive for a light reflecting film in a double-sided recording MO.

The effect of the present invention is expected when the layer is indirect contact with different kinds of material has the specificconductivity of the immersion water within a specified range, regardlessof whether the layer is based on a hot-melt adhesive or based on a curedproduct (cured film) of an ultraviolet curable composition. The presentinvention, which uses an ultraviolet curable composition as an adhesive,clearly has superior effects in that the present invention can satisfythe requirements in properties specific to DVDs such as the requirementof small curling even if thermal hysteresis is present at the time inthe bonding or after the bonding, and that it can further increase theproductivity of bonding per unit hour.

EMBODIMENTS OF THE INVENTION

Embodiments of the present invention will be described in detail basedon preferred embodiments of the present invention.

First, one disc-like plastic substrate in which a 40-60 nm metal thinfilm is laminated on irregularities called pits, corresponding to therecorded information, and one disc-like plastic substrate in which a10-30 nm semitransparent thin film of gold is laminated onirregularities called pits, corresponding to the recorded information,are prepared.

Then, an ultraviolet curable composition is prepared such that water inwhich the cured coating film has been immersed has a specificconductivity of 100 μS/cm or less and the water absorption of the curedcoating film is 1 to 7 wt %, using two kinds or more of polyfunctional(meth)acrylate having two or more (meth)acryloyl groups as apolymerizable monomer or polymerizable oligomer, and optionally amonofunctional monomer, as needed, and further using photopolymerizationinitiators in an amount of 2 to 7 parts by weight per 100 parts byweight of the liquid ultraviolet curable composition, one of thephotopolymerization initiators having an absorption wavelength in theregion of 370 nm or higher and having a molecular extinction coefficientof 50 (l/mol.cm) or higher in a wavelength region of 370 nm to 450 nm,and another photopolymerization initiator being of a molecule cleavingtype or a hydrogen eliminating type, which is not encompassed by therange of the former photopolymerization initiator.

The above-described composition is coated on the metal thin film surfaceof a 40-60 nm metal thin film-laminated disc-like plastic substrate anda 10-30 nm gold semitransparent film-laminated disc-like plasticsubstrate is bonded such that the semitransparent film surface isopposed to the metal thin film surface. Ultraviolet light is irradiatedfrom one side or both sides of the bonded disc to unite the two to forma DVD-9 disc.

EXAMPLES

Next, the present invention will be described in detail by examples.However, the present invention is not limited to these examples. In theexamples, “parts” indicates “parts by weight.”

Example 1

Twenty six (26) parts of urethane acrylate obtained by reacting 1 moleof polytetramethylene glycol (molecular weight 850) and 2 moles ofisophorone diisocyanate, and then with 2 moles of hydroxyethyl acrylate,24 parts of 2-hydroxy-3-phenoxypropyl acrylate, 17 parts of UNIDICKV-5500 (manufactured by DAINIPPON INK AND CHEMICALS, INC.) as bisphenolA type epoxyacrylate, 20 parts of ethyl carbitol acrylate, 1.5 parts ofethylene oxide-modified trimethylolpropane triacrylate, 0.2 part ofethylene oxide-modified phosphoric acid methacrylate, 0.3 part of ethyldimethylaminobenzoate, 5 parts of 2-hydroxypropyl acrylate, 2 parts of2,4,6-trimethylbenzoyldiphenylphosphine oxide as a photopolymerizationinitiator, and 4 parts of 1-hydroxycyclophenyl ketone were mixed anddissolved at 0° C. for 1 hour to prepare a pale yellow transparentultraviolet curable composition. Using this composition, the durabilityof a DVD-9 bonded disc, the specific conductivity of water in which thecured coating film has been immersed, and the water absorption of thecured coating film were evaluated by the following Test Methods 1, 2 and3, respectively. The results are shown in Table 1.

Example 2

Fifteen (15) parts of urethane acrylate obtained by reacting 1 mole ofpolytetramethylene glycol (molecular weight 850) and 2 moles ofisophorone diisocyanate, and then with 2 moles of hydroxyethyl acrylate,22 parts of 2-hydroxy-3-phenoxypropyl acrylate, 20 parts of UNIDICKV-5500 (manufactured by DAINIPPON INK AND CHEMICALS, INC.) as bisphenolA type epoxyacrylate, 12 parts of trimethylolpropane triacrylate, 5parts of tripropylene glycol diacrylate, 17 parts of tetrahydrofurfurylacrylate, 1.5 parts of ethylene oxide-modified trimethylolpropanetriacrylate, 0.2 part of ethylene oxide-modified phosphoric acidmethacrylate, 0.3 part of ethyl dimethylaminobenzoate, 2 parts of2-hydroxypropyl acrylate, 2 parts of2,4,6-trimethylbenzoyldiphenylphosphine oxide as a photopolymerizationinitiator, and 4 parts of 1-hydroxycyclophenyl ketone were mixed anddissolved at 60° C. for 1 hour to prepare a pale yellow transparentultraviolet curable composition. Using this composition, the durabilityof a DVD-9 bonded disc, the specific conductivity of water in which thecured coating film has been immersed, and the water absorption of thecured coating film were evaluated by the following Test Methods 1, 2 and3, respectively. The results are shown in Table 1.

Comparative Example 1

Twenty two (22) parts of urethane acrylate obtained by reacting 1 moleof polytetramethylene glycol (molecular weight 850) and 2 moles ofisophorone diisocyanate, and then with 2 moles of hydroxyethyl acrylate,23 parts of 2-hydroxy-3-phenoxypropyl acrylate, 20 parts of DICKLIGHTUE-8200 (manufactured by DAINIPPON INK AND CHEMICALS, INC.) as bisphenolA type epoxyacrylate, 23 parts of ethyl carbitol acrylate, 4.5 parts ofethylene oxide-modified trimethylolpropane triacrylate, 0.2 part ofethylene oxide-modified phosphoric acid methacrylate, 0.3 part of ethyldimethylaminobenzoate, 1 part of hydroxyethyl methacrylate, 2 parts of2,4,6-trimethylbenzoyldiphenylphosphine oxide as a photopolymerizationinitiator, and 4 parts of benzyl dimethyl ketal were mixed and dissolvedat 60° C. for 1 hour to prepare a pale yellow transparent ultravioletcurable composition. Using this composition, the durability of a DVD-9bonded disc, the specific conductivity of water in which the curedcoating film has been immersed, and the water absorption of the curedcoating film were evaluated by the following Test Methods 1, 2 and 3,respectively. The results are shown in Table 1.

<Test Method 1: Durability Test on DVD-9 Bonded Disc>

On a polycarbonate disc on which pits for recording information isformed and aluminum is laminated to 50 nm, the above-describedcomposition was coated with a dispenser, and this polycarbonate disc wasoverlaid with a polycarbonate disc on which gold is laminated to 15 nmas a semitransparent film. Then using a spin coater, the discs wererotated at 1,500 rpm for 5 to 6 seconds so that the film thickness ofthe cured coating film became about 50-60 μm. Then, using an ultravioletirradiation apparatus of a belt-conveyor type equipped with a 120 W/cmmetal halide lamp (EYE GRAPHICS CO., LTD. M03-L31, with a cold mirror)adjusted at a lamp height of 10 cm, irradiation of 1,000 mJ/cm² was madein air from the side of the substrate with the gold semitransparent filmto make a DVD-9 type bonded disc. On this occasion, the amount ofirradiation irradiated to the adhesive transmitted through the substratewith the gold semitransparent film was 500 mJ/cm². The ultravioletactinometer used was a UV Meter (UVPF-36) manufactured by EYE GRAPHICSCO., LTD.

Durability tests were conducted in which the bonded disc was left in ahigh temperature and high humidity environment of 80° C. and 95% RH for96 hours and 300 hours, respectively, and evaluation of the signalproperties before and after the durability test were performed. Thedurabilities obtained were compared by the ratios of error rates beforeand after the test (after the durability test/before the durabilitytest). The results are shown in Table 1. If there is no change in theerror rate, this value is 1. Larger values indicate worse durability ofthe bonded disc.

<Test Method 2: Measurement of Specific Conductivity of Water in WhichCured Coating Film Has Been Immersed>

The specific conductivity was measured by the above-described procedure.As a reference, a polymer bottle was provided in which only pure waterwas charged and measured initially and after the test. As a result, theinitial value was 1.4 μS/cm and after the test it was 1.6 μS/cm, so thatno effect from the container was observed.

<Test Method 3: Measurement of Water Absorption of Cured Coating Film>

The ultraviolet curable composition was coated on a glass plate to afilm thickness of about 100 μm.

Then, under a nitrogen atmosphere, using as a UV lamp an M03-L31 (120W/cm metal halide lamp, with a cold mirror) manufactured by EYE GRAPHICSCO., LTD., irradiation of 500 mJ/cm² (an ultraviolet actinometer:UVPF-36, manufactured by EYE GRAPHICS CO., LTD.) at a lamp height of 10cm was carried out to prepare a cured coating film. The water absorptionof the cured coating film was measured according to the Method B of JISK7209-1984.

Example 3

Using the same composition as in Example 1, the durability of a DVD-9bonded disc, the specific conductivity of water in which the curedcoating film has been immersed, and the water absorption of the curedcoating film were evaluated by the following Test Methods 4, 5 and 6,respectively. The results are shown in Table 1.

Example 4

Using the same composition as in Example 2, the durability of a DVD-9bonded disc, the specific conductivity of water in which the curedcoating film has been immersed, and the water absorption of the curedcoating film were evaluated by the following Test Methods 4, 5 and 6,respectively. The results are shown in Table 1.

Comparative Example 2

Using the same composition as in Comparative Example 1, the durabilityof a DVD-9 bonded disc, the specific conductivity of water in which thecured coating film has been immersed, and the water absorption of thecured coating film were evaluated by the following Test Methods 4, 5 and6, respectively. The results are shown in Table 1.

<Test Method 4: Durability Test on DVD-9 Bonded Disc>

On a polycarbonate disc on which pits for recording information isformed and aluminum is laminated to 50 nm, the above-describedcomposition was coated with a dispenser, and this polycarbonate disc wasoverlaid with a polycarbonate disc on which gold is laminated to 15 nmas a semitransparent film. Then using a spin coater, the discs wererotated at 1,500 rpm for 5 to 6 seconds so that the film thickness ofthe cured coating film became about 50 to 60 μm. Then, using a flashirradiation apparatus equipped with 4 lamps, adjusted such that theinput energy per shot per lamp is 200 J, ultraviolet light wasirradiated at a frequency of 2 Hz by 10 shots form the side of thesubstrate with the gold semitransparent film to prepare a DVD-9 bondeddisc. The emission spectrum of flash irradiation light from the lamp wasmeasured using a spectroradiometer (USR-20A manufactured by Ushio Inc.)and integrated from 300 nm to 390 nm to obtain the ultraviolet lightstrength and irradiation amount, which were adjusted such that theultraviolet light strength was 19 W/cm² and the ultraviolet irradiationamount was 12 mJ/cm² with the sum of 10 shots being 120 mJ/cm².

Durability tests were conducted in which the bonded disc was left in ahigh temperature and high humidity environment of 80° C. and 95% RH for96 hours and 300 hours, respectively, and evaluation of the signalproperties before and after the durability test were performed. Thedurabilities obtained were compared by the ratios of error rates beforeand after the test (after the durability test/before the durabilitytest). The results are shown in Table 1. If there is no change in theerror rate, this value is 1. Larger values indicate worse durability ofthe bonded disc.

<Test Method 5: Measurement of Specific Conductivity of Water in WhichCured Coating Film Has Been Immersed>

The specific conductivity was measured by the above-described procedure.As a reference, a polymer bottle was provided in which only pure waterwas charged and measured initially and after the test. As a result, theinitial value was 1.4 μS/cm and after the test it was 1.6 μS/cm, so thatno effect from the container was observed.

<Test Method 6: Measurement of Water Absorption of Cured Coating Film>

The ultraviolet curable composition was coated on a glass plate to afilm thickness of about 100 μm.

Then, under a nitrogen atmosphere, using a flash irradiation apparatus,adjusted such that input energy per shot per lamp was 200 J, ultravioletlight was irradiated at a frequency of 2 Hz by 10 shots from the side ofthe substrate with the gold semitransparent film to prepare a curedcoating film. The water absorption of the cured coating film wasmeasured according to the Method B of JIS K7209-1984.

TABLE 1 Comparative Example 1 Example 2 Example 1 Comparative ContinuousContinuous Example 3 Example 4 Continuous Example 2 light light FlashFlash light Flash Specific conductivity of 79 62 46 35 163 115 water inwhich cured coating film has been immersed (μS/cm) Water absorption ofcured 1.8 1.5 1.9 2.0 1.6 1.6 coating film (wt %) DVD-9 Durability(Error rate increase rate) 80°C. 95% RH  96 H 9 4 5 3 200 190 300 H 2520 9 5 Impossible to Impossible to measure measure

The measurement of the specific conductivity of water in which the curedcoating film has been immersed was performed by forming a cured coatingfilm of composed of cured product of the composition of the presentinvention on a glass plate and conducting the measurement using this andshowing the results obtained. The results were equivalent to themeasured values of specific conductivity obtained by conductingmeasurement similarly after peeling the cured coating film between thedifferent kinds of material form the actually bonded optical disc.

For the compositions of Examples 1 to 4, water in which the curedcoating film has been immersed had a specific conductivity of 100 μS/cmor less, and as a result of durability tests on DVD-9 types bonded discsusing a gold semitransparent film, each example showed substantially nochange in signal properties and exhibited excellent durability (in thecase of 96 hours). Further, Example 3 and Example 4 showing 50 μS/cm orless showed substantially no change in signal properties after 300hours.

A DVD-9 disc using the composition of the present invention, when leftat room temperature after a high temperature and high humidity test at80° C. and 95% RH 96H, generated no bubbles and caused no reproductionfailure on a player.

INDUSTRIAL APPLICABILITY

The composition of the present invention contains no or minimal amountsof components which can be water-soluble electrolyte components in itscured coating film and as a result water in which the cured coating filmhas been immersed has a specific conductivity lower than conventionalcompositions, so that its use enables production of DVD-9 discs whichare excellent in disc durability and are highly reliable.

What is claimed is:
 1. An ultraviolet curable composition for opticaldiscs, the composition containing an ultraviolet curable compound and aphotopolymerization initiator and being used when bonding two substrateswhich have information recording layers, respectively, on each outermostlayer of which a thin film of different type of material is formed,wherein the ultraviolet curable composition is characterized in thatwater in which a cured coating film has been immersed has a specificconductivity of 100 μS/cm or less.
 2. An ultraviolet curable compositionfor optical discs, the composition containing an ultraviolet curablecompound and a photopolymerization initiator and being used as aprotective coat for protecting at least one thin film of two substrateswhich have information recording layers, respectively, on each outermostlayer of which a thin film of different type of material is formed,wherein the ultraviolet curable composition is characterized in thatwater in which a cured coating film has been immersed has a specificconductivity of 100 μS/cm or less.
 3. The composition as claimed inclaim 1, wherein the water absorption of the cured coating film is 1 to7 wt %.
 4. The composition as claimed in claim 1, wherein as theultraviolet curable compound is used a polyfunctional (meth)acrylatehaving two or more (meth)acryloyl groups.
 5. The composition as claimedin claim 4, wherein as the photopolymerization initiator is used1-hydroxycyclohexyl phenyl ketone.
 6. The composition as claimed inclaim 1, 3, 4, or 5, wherein the water in which the cured coating filmhas been immersed has a specific conductivity of 50 μS/cm or less. 7.The composition as claimed in claim 1, 3, 4, or 5, wherein thecombination of different kinds of material is a combination of a metalcomposed mainly of aluminum and gold.
 8. The composition as claimed inclaim 1, 3, 4, or 5, wherein the combination of different kinds ofmaterial is a combination of a metal composed mainly of aluminum and asilicon based inorganic compound.
 9. An optical disc having at least abonded structure comprising two substrates having information recordinglayers, respectively, on each outermost layer of which a thin film ofdifferent type of material is formed, the thin films in the respectivesubstrates being bonded to each other through a cured coating film of anultraviolet curable composition containing an ultraviolet curablecompound and a photopolymerization initiator, wherein the optical discis characterized in that water in which the cured coating film has beenimmersed has a specific conductivity of 100 μS/cm or less.
 10. Anoptical disc having at least a bonded structure comprising twosubstrates having information recording layers, respectively, on eachoutermost layer of which a thin film of different type of material isformed, on one or each of the thin films a protective coat layercomposed of a cured coating film which transmits to ultraviolet lightbeing provided, the coating film being obtained from an ultravioletcurable composition containing an ultraviolet curable compound and aphotopolymerization initiator, the protective coat layer of one of thesubstrates and the thin film of the other of the substrates, or theprotective coat layers of both substrates, being bonded together by thecured coating film of the ultraviolet curable composition containing anultraviolet curable compound and a photopolymerization initiator,wherein the optical disc is characterized in that water in which thecured coating film of the protective coat layer has been immersed has aspecific conductivity of 100 μS/cm or less.
 11. The optical disc asclaimed in claim 9, wherein the water absorption of the cured coatingfilm is 1 to 7 wt %.
 12. The optical disc as claimed in claim 9, whereinthe cured coating film is a cured coating film containing a curedproduct of a polyfunctional (meth)acrylate having two or more(meth)acryloyl groups.
 13. The optical disc as claimed in claim 9, 11,or 12, wherein the water in which the cured coating film has beenimmersed has a specific conductivity of 50 μS/cm or less.
 14. Theoptical disc as claimed in claim 9, 11, or 12, wherein the combinationof different kinds of material is a combination of a metal composedmainly of aluminum and gold.
 15. The optical disc as claimed in claim 9,11, or 12, wherein the combination of different kinds of material is acombination of a metal composed mainly of aluminum and a silicon basedinorganic compound.
 16. A method for producing an optical disc,comprising providing an ultraviolet curable composition containing anultraviolet curable compound and a photopolymerization initiator, suchthat water in which an ultraviolet transmitting cured coating film hasbeen immersed has a specific conductivity of 100 μS/cm or less, betweenthin films on two substrates comprising information recording layers, oneach outermost layer of which the thin film of different type ofmaterial is formed; arranging the thin films to oppose each other;bonding them to each other; and then irradiating ultraviolet light tothe composition through either one of or both of the thin films toconvert the composition into a cured coating film to integrate bothsubstrates with each other.
 17. The production method as claimed inclaim 16, wherein as the ultraviolet curable compound is used apolyfunctional (meth)acrylate having two or more (meth)acryloyl groups.18. The production method as claimed in claim 16 or 17, wherein theultraviolet irradiation is by continuous light irradiation.
 19. Theproduction method as claimed in claim 16 or 17, wherein the ultravioletirradiation is by flash irradiation.
 20. A method for producing anoptical disc, comprising providing an ultraviolet curable compositioncontaining an ultraviolet curable compound and a photopolymerizationinitiator, such that water in which an ultraviolet transmitting curedcoating film has been immersed has a specific conductivity of 100 μS/cmor less, on at least one or both of thin films each of which is formedon one of two substrates comprising information recording layers, oneach outermost layer of which the thin film of different type ofmaterial is formed; irradiating ultraviolet light onto the compositionto provide a protective coat layer composed of a cured coating film ofthe ultraviolet curable composition; then providing an ultravioletcurable composition containing an ultraviolet curable compound and aphotopolymerization initiator between the protective coat layer of onesubstrate and the thin film of the other substrate opposing each other,or between the protective coat layers of both substrates opposing eachother; bonding them together; and irradiating ultraviolet light througheither one of or both of the thin films to convert the composition intoa cured coating film to integrate both substrates with each other. 21.The method for producing an optical disc, comprising bonding two opticaldiscs produced as claimed in claim 16 or 20 such that the one of thethin films in the respective discs having a higher ultraviolettransmittance is arranged on the outer side.